Gravity Fed System Air Vents

The 1100 liter tank on the roof that I will be useing to gravity-feed my house has 2 1/4" vent holes in the top of the screw-on cap. I will be coming out of tank with a 1" pipe going in several directions across the roof to 90 down the walls to the various faucets. Should I put in some intermediate vent tubes along the system as well?

Absolutely no reason to do it. The vents are allow water to enter and leave the tank. Once the tank has water in it, air will not get into any of the piping, UNLESS you add vents so it can happen, and that would not be a good thing.

Sorry about that. EVERY house in Mexico has vent tubes coming up from at least one of the 90s going down into the walls. Some houses may have 5 or 6 vents sticking up as high as necessary to clear the level of the top of the tank-- really an eyesore. And, EVERY 'plumber' down here will tell you that the vents are absolutely necessary for max pressure in these gravity-fed systems.

I have been looking at a type of pressure booster pump that fits right at the outlet of the roof tank. It is no bigger than a small electric motor. It only comes on when water starts moving through it. The power is subject to go off at any given time through the day (viva Mexico!!) and I am not sure if full water pressure/volume will still pass through the thing when the power is off.

What is often done in the USA if the pressure is low, is to add a small bladder tank with a pump. This is very similar to what a well uses. The bladder tank is filled with water under pressure by the pump. It has a range, it gets pumped up to say 60psi then stops. This allows you to draw some water for say washing your hands without having to have the pump come on for every small water usage. As the pressure in the bladder tank drops, often to around 40psi, the pump turns back on to to refill the tank under pressure or to maintain it for a sustained use. If the electricity were to go off, you'd still have a small amount of water in the bladder tank under pressure, and once that was used up, water should pass through from your storage tank. This video shows one example of what I'm talking about http://www.thisoldhouse.com/toh/video/0,,20305378,00.html

The ONLY thing that gives you "pressure" is the "height" of the water above the faucet. Addional vents on the water system are COMPLETELY USELESS, regardless of what "Everyone says". The only thing that happens with those vents is that the water goes up inside them to the same level as the water in the tank, but that does nothing to "increase" the pressure. If you want to spend the money, A Y McDonald makes a "booster pump" which operates when the water starts flowing. It does NOT have to be at the tank but should be somewhere before the first faucet to give pressure everywhere. It is installed with a bypass so the water can flow normally, if necessary.